Australian researchers observed that the e-nose for chicken odor worked in-shed yet was unreliable beyond the shed . Today’s e-noses function well for the context for which they are designed but do not yet cover broad environmental odor monitoring . A workgroup in Europe was launched in 2015 to develop a standard for e-noses . One area under development is stack monitoring, where the conditions are more predictable than ambient monitoring yet harsh on the equipment. Producing minimum performance standards and other essential criteria will help guide the field. Unlocking the molecular features that trigger our sense of smell may someday lead to improved e-noses. Keller et al. supplied chemoinformatic data and sensory data on 407 molecules to teams so they could develop predictive algorithms. The algorithms were tested on 69 molecules, and the results were favorable for 8 odor notes out of 19 total. With successful reverse-engineering of the smell of a molecule and then combining that with appropriate sensors, a true e-nose that fully mimics the human nose may be achieved some day. Other technologies have adopted the “e-nose” name, such as portable, fast gas chromatographs or mass spectrometers . Even “electronic mucosa” is under development. In the nasal cavity, natural mucosa acts like the stationary phase of a gas chromatography column to differentially apportion odorants. “Electronic mucosa” consists ofmultiple sensor arrays,indoor grow light shelves each separated by gas-chromatograph-like micro columns. The rich data set obtained could predict the presence of odorants at low concentrations .
No single approach can successfully address nuisance odor complaints . Human panels provide some of the strongest information yet, due to the variable perception of odors, yield inconsistent results. Chemical analysis provides confirmation of exposure to specific odorants yet may miss the key odorants. Instead of a single method, the right mix of sensory and analytical methods needs to be used . The Odor Profile Method followed by GC-sensory conformation provides one of the strongest tools today. The use of standard odorants to calibrate panelists has been advocated since the 1970s . The use of n-butanol, however, has not led to transferability of results to non-butanol odors according to a review of 412 odor measurements . In Denmark, both n-butanol and hydrogen sulfide are used to calibrate panelists, and the Japanese method uses five odorants in its panelist aptitude test . The use of multiple odorant standards may have advantages. Observations from related fields provide insights for environmental odor exposure assessment. Setting odor source minimum-distance setbacks and emission rates did not take into account the level of nuisance , so complaints continued . For the drinking water industry, analytical measurements and dilution-to-threshold limits did not resolve underlying taste-and-odor problems and correlated poorly with customer complaints .To save resources, risk assessment uses a screening-level assessment before a refined risk assessment is performed. The same approach works for odor exposure assessment and is practiced widely already. Two air inspectors, ideally, respond to a nuisance odor complaint so they can corroborate the complainant’s and each others’ sense. If corroboration occurs, the source of the odor is usually identified by the complainant or the air inspectors’ prior knowledge of the area.
Standard practice is to move upwind of the identified source to confirm that the odor is not coming from elsewhere. Recording wind direction, details of the complaint and any action taken document the investigation.Training air quality inspectors on how to better identify describe odors might assist their work. For example, using an odor wheel and maintaining a log of complaints could identify trends. For many air districts, however, the ability to sense “something” in the air at the complainant’s location is sufficient. The odor intensity only need be noted as “faint” or “strong” to help prioritize the nuisance. Matching the complainant’s vocabulary of odor is unnecessary. It is necessary for the air inspector’s sense of smell to be verified periodically. An anosmic air inspector, obviously, would be unfit for odor patrols. For difficult-to-discern odors, returning to the location several times may be required. Involving a panel of air inspectors should help detect odors that are especially low. The increased cost of a panel, and its training, would need to be taken into account.When a screening-level assessment fails to address an odor nuisance, more refined exposure assessment is necessary. Complexities such as overlapping odors, unknown sources and resistance on the part of the potential source to acknowledge responsibility can also require refined assessments. Refined assessments require more sophisticated documenting of the odor. The field assessments for plumes is a logical starting point. More subtle, ongoing odors require the grid approach . The various odor wheels included in Appendix A, in addition to the list used in Europe, help form a standard odor lexicon.
Meteorological data factors into both of these odor-documentation techniques.If enough information on individual odorants within the environmental odor is available, such as through prior sampling of similar sources, then sampling of the odorous air may provide insights. Residential sampling using evacuated canisters has been successful, as long as container purges are performed and blanks are run in parallel. Trace analysis requires such. If air sampling bags are used, quality control samples must be included. Bag materials have been shown to adsorb certain odorants, allow them to escape, or sometimes contribute compounds to the odorant load. Figure 3.9 shows results for various bag materials spiked with ethylene . Greater adsorption has also been observed for certain odorants in Tedlar™ bags as compared to Teflon™ bags .Currently there is no international standard on how to respond to nuisance odor complaints, not even straightforward screening-level steps. International organizations such as Organization for Economic Cooperation and Development and World Health Organization focus more on health effects that nuisance complaints. Even fundamental sampling and analysis techniques have no agreed upon standard for environmental odors in ambient air. National and regional agencies have filled these gaps spottily. Once standards have been developed, they become entrenched and resistant to new, improved techniques. There is a clear need to develop standard methods for environmental odor monitoring in ambient air, combining both sensory and analytical methods. WHO recommends that future focus on perception of the actual odor rather than measuring individual odorants . Population dose-response measurements based on field studies are needed to build a fundamental knowledge base. Measures of “population annoyance” need to be developed, which Germany, the Netherlands and New Zealand are working toward . Quality standards, such as field blanks and spiked samples, are routine in air quality sampling and must be implemented in odor studies. Otherwise sample degradation, odorant loss or the introduction of unintended trace odorants is unknown. A compendium of sampling materials and their performance for specific odorants is needed. Basic research on odors will help advance the predictive power of analytical measurements. Investigating and eventually being able to predict the synergistic, antagonistic, or masking interactions among odorants is especially important. Temperature and humidity effects are also a challenge requiring more research. The expansion of GC-sensory techniques and increased sensitivity, including greater use of SIFT-MS,rolling tables will likely bridge the gap between sensory and analytical detection that exists today.Due to the complex physiological and psychological factors involved in human olfaction, the prediction of odor impact by e-noses will probably remain allusive. Nonetheless, closer approximation of human olfaction by sophisticated sensor arrays may reduce the reliance on human panels and offer continuous monitoring in the future. Machine learning is already being used to decipher human olfactory responses through the study of pattern-based odor detection and recognition, olfactory phenotypes, disease biomarkers, physicochemical properties that predict olfaction, and public database mining .The current approach to environmental odor exposure is to consider the complaint in-and of itself as the initial concern. Development of odor evaluations tools such as the OPM, which uses odor wheels and panel evaluation of odor note and odor intensity, may be used in concert with quantitative analytical methods so odor can be objectively legislated and monitored . A range of instrument and sensory techniques are available, from sensors to questionnaires, yet no standard approach is followed. The best advice from various reviews is that a mix of techniques is required to address nuisance odor complaints . Currently, two diverging paths are followed. One path leads toward greater population based metrics of odor, such as odor diaries and visiting grid points multiple times a year to assess an odor. Using a panel to evaluate the extent of a plume falls into this category as well. Although the number of studies using OPM grew over the past decade, a larger set would add maturity to the method and greater acceptance. The proven track record in drinking water assessment is promising. The qualitative odor-note evaluation and an expanded list of suspect odorants holds great potential. The other trend is toward increasingly sophisticated analytical techniques.
The expansion of GC-sensory techniques and increased sensitivity, including greater use of SIFT-MS, will likely bridge the gap that exists today between sensory and analytical detection. Mimicking the human sense of smell remains an aspiration. Although not field-worthy for stack or ambient sampling at this time, the advances in machine learning and bio-mimicry could make such sensor systems a reality in the future.Risk assessment has its roots in the 1970s and in the 1980s began to bring order to an unmanageable amount of data on toxic chemicals and radiation in the hope to help regulators make better decisions . Through the 1990s, the field matured, especially under pesticide and Superfund site-remediation programs. Since 1983, the National Research Council has updated risk assessment guidance regarding the science, decision-making, and communication of risk , and the field continues to evolve. Risk assessment has broad acceptance in the United States and other countries, including Canada, Australia and Europe.A bit of confusion has occurred with the term “risk assessment.” Too often it is misused to mean just the estimation of the low-dose response following exposure to a carcinogenic chemical. That is the dose-response assessment, just one component of risk assessment. The term “risk assessment” is often confused with “risk management” as well. Risk assessment provides useful input to risk management but does not supplant it. Risk management involves policy, values and communication steps that go well beyond risk assessment . As practiced, risk assessments vary widely in scope and purpose. Some assess a single chemical across a range of exposure scenarios while others are site-specific and assess the variety of chemicals found at that particular site. In general, risk assessment is used to evaluate involuntary exposures. Voluntary risks are generally accepted by the public and receive less scrutiny .Risk assessment is central to this paper and will be applied through case studies rather than an assessment of a specific odor at a specific location using previously unpublished field data. Information on the health risks posed by odors, such as the case studies, was gathered through a literature search. As a starting point, a previous risk assessment of odorants was augmented by a substantial review of the human health effects of odors by Alberta Health in Alberta, Canada . The 216-page review by Alberta Health included literature through July 2013. The present literature search was for documents published after this date, and the key findings of Alberta Health will be noted. The starting point for risk assessment methodology was the foundational work by the National Research Council , now updated for the 21st century . The programs that grew out of the original work have issued their own guidance, which was also consulted. Such programs include pesticides , site remediation , exposure factors and California-specific work . International efforts, such as the review of the chemicals in commerce in the European Union , were also consulted. The 1,556-page tome edited by Dennis Paustenbach titled “Human and Ecological Risk Assessment: Theory and Practice” was an additional starting point. The focus was post-2013, and when relevant articles or books were found the “cited by” function was used to find even more up-to-date information. Reviews of the health risks from odors were especially sought. Government and health agency websites were searched as well. Alberta Health critically evaluated over 500 peer-reviewed epidemiology and experimental studies on the impacts of odors on human health .